net: dsa: Add basic framework to support ndo_fdb functions

Provide callbacks for ndo_fdb_add, ndo_fdb_del, and ndo_fdb_dump.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>